This is the problemther e-readers correctly display the text as intended, whereas Sygil's Book View display many extra spaces between the lines, thus becoming useless for preview purposes. All the other properties are respected.

Maybe there's something I need to know about Sygil's behavior on Book View mode? What am I doing wrong?

This is the problemther e-readers correctly display the text as intended, whereas Sygil's Book View display many extra spaces between the lines, thus becoming useless for preview purposes. All the other properties are respected.

Maybe there's something I need to know about Sygil's behavior on Book View mode? What am I doing wrong?

Sigil is not a page viewer. It scrolls the page unlike eBooks. Percentages won't work as values since there is no height to the page. You need to use better values. How do you know how big the page is on the eBook? Margins are better set it ems rather than percentages of some unknown size page.

Percentages in vertical margins are supposed to be referred to the element's width instead of height (so that margin-top: 5% and margin-right: 5% should be the same size). But of course, the implementation could be wrong in Sigil or in other readers.